Aromatic polyimide materials are generally prepared by the reaction of an organic diamine with a tetracarboxylic acid dianhydride to form a polyamide acid, with subsequent conversion of the polyamide acid to a polyimide. Techniques for the preparation of such polymers are found, for example in Endrey, U.S. Pat. No. 3,179,631, British Pat. No. 981,543, and Gall, U.S. Pat. No. 3,249,588. The Endrey patent involves simultaneously converting the polyamide acid to the polyimide and precipitating the polymer from solution. An alternative technique previously suggested and illustrated in example 7 of the Endrey patent involves first precipitating the polyamide acid and subsequently converting the polyamide acid to polyimide by thermal or chemical means. This results in resins having low crystallinity and low surface area. The processes shown in the Gall patent result in polyimides having high surface area and high crystallinity.
Polyimides are used industrially in a wide variety of applications. For example, polyimides can be formed into a shaped article such as a film, or can be compounded into a coating enamel. Still other uses for such resins are in molding applications, in which polyimide in particulate form is fabricated into various configurations that can be used in a variety of technically demanding environments such as jet engines, business machines, automotive components and diverse industrial equipment. Such molded polyimide parts are capable of withstanding high temperatures and exhibit excellent bearing properties, good electrical properties and excellent creep resistance. However, continuing effort has been directed to the improvement of the mechanical properties of these resins, such as toughness, which would permit their use in a still broader range of high temperature environments.